In a typical cellular radio system or a radio communications network, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a radio access network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks also may be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Packet System (EPS) have been completed within the 3GPP and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base stations are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations, e.g., eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
Recently two main trends have emerged in the cellular telephony business. First mobile broadband traffic is more or less exploding in e.g. the WCDMA networks. The technical consequence is a corresponding steep increase of the interference in these networks, or equivalently, a steep increase of the load. This makes it important to exploit the load headroom that is left in the most efficient way. Secondly, radio communications networks are becoming more heterogeneous, with macro radio base stations being supported by micro radio base stations at traffic hot spots. Furthermore, WCDMA home base stations, also called femto radio base stations, are emerging in many networks. This trend clearly puts increasing demands on inter-cell interference management.
Below it is described the measurement and estimation techniques, needed to measure the instantaneous total load, also referred to as the Rise over Thermal, RoT, on the uplink air interface.
WCDMA Load
The Need for Accurate Load Estimation
The air interface load of the WCDMA uplink is a fundamental quantity for                the scheduling, in the RBS, of Enhanced Uplink (EUL) user equipments.        the admission and congestion control algorithms, in the RNC, that also control        
the load created by release 99 user equipments.
There are several reasons for this. Firstly, the fast inner power control loop coupling between user equipments can create instability if too much load is allowed in the uplink, the so called party effect. Such power rushes originate e.g. when a new high power user equipment is entering the uplink causing interference. Since the inner loop power control strives to maintain the Signal to Interference Ratio (SIR) at a specified level, the consequence is that the other user equipments of the cell increase their power, which in turn increases the interference and lead to additional power increases. At a certain point this process goes unstable with unlimited power increases of all user equipments in the uplink of the cell.
Secondly, it is well known that increased interference levels reduce the coverage, simply because a terminal or user equipment needs to transmit with a higher power to overcome an increased interference level. At the cell boundary, the UE power is hence saturated, meaning that the user equipment must move towards the base station, in case of interference, to be detected—hence the cell size is reduced.
Thirdly, the scheduling of enhanced uplink user equipments in the RBS does not account for release 99 legacy traffic from user equipments that do not support EUL. Even modern user equipments may lack support for EUL. In order to keep the air interface load under control, release 99 traffic must hence be monitored elsewhere. In WCDMA this control functionality is performed in the RNC, by the admission and congestion control algorithms. Since the consequences, instability and loss of coverage, are the same as for EUL user equipments when the air interface becomes over-utilized, it follows that also the admission and congestion control algorithms need to have access to a measure of the momentary air interface load.
Finally, it is crucial that the load measure is accurate. This follows since the load, which is expressed as a (noise) Rise over Thermal, see below, is usually limited to be below 10-15 dB. It also follows that any load estimation errors will require margins that reduce the limit of 10-15 dB to lower values, a fact that will reduce the cell capacity. Hence all quantities that are used to form the uplink air-interface load need to be estimated very accurately, say at a 0.1-0.2 dB level so as not to limit uplink mobile broadband performance.
Problems in WCDMA Load Estimation
It is well known that the load at the antenna connector of the WCDMA uplink is given by the noise rise, or Rise over Thermal, RoT(t), defined by
                                          RoT            ⁡                          (              t              )                                =                                    RTWP              ⁡                              (                t                )                                                                    N                0                            ⁡                              (                t                )                                                    ,                            eq        .                                  ⁢                  (          1          )                    where N0(t) is the thermal noise level, also referred to as the noise floor level, the noise power floor level or the thermal noise power floor, as measured at the antenna connector. ‘t’ denotes the time. It remains to define what is meant by RTWP(t). The definition used here is simply the total wideband power RTWP(t)
                                          RTWP            ⁡                          (              t              )                                =                                                    ∑                                  k                  =                  1                                K                            ⁢                                                P                  k                                ⁡                                  (                  t                  )                                                      +                                          I                N                            ⁡                              (                t                )                                      +                                          N                0                            ⁡                              (                t                )                                                    ,                            eq        .                                  ⁢                  (          2          )                    also measured at the antenna connector. Here IN(t) denotes the power as received from neighbour cells (N) of the WCDMA system, and Pk(t) is the power of the k:th user of the own cell. As will be seen below, the major difficulty of any RoT estimation algorithm is to separate the thermal noise power floor from the interference from neighbor cells.
Another specific problem that needs to be addressed is that the signal reference points are by definition at the antenna connector. The measurements are however obtained after the analogue signal conditioning chain, in the digital receiver. The analogue signal conditioning chain does introduce a scale factor error of about 1-3 dB, 1-sigma, that is difficult to compensate for. Fortunately, all powers of eq. (2) are equally affected by the scale factor error so when eq. (1) is calculated, the scale factor error is cancelled as
                                          RoT            DigitalReceiver                    ⁡                      (            t            )                          =                                                            RTWP                DigitalReceiver                            ⁡                              (                t                )                                                                    N                DigitalReceiver                            ⁡                              (                t                )                                              =                                                                      γ                  ⁡                                      (                    t                    )                                                  ⁢                                                      RTWP                    Antenna                                    ⁡                                      (                    t                    )                                                                                                γ                  ⁡                                      (                    t                    )                                                  ⁢                                                      N                    Antenna                                    ⁡                                      (                    t                    )                                                                        =                                                            RoT                  Antenna                                ⁡                                  (                  t                  )                                            .                                                          eq        .                                  ⁢                  (          3          )                    
The superscripts DigitalReceiver and Antenna indicate quantities valid at the digital receiver and the antenna respectively, and γ(t) denotes said scale factor error. In order to understand the fundamental problem of neighbor cell interference when performing load estimation, note thatIN(t)+N0(t)=E└IN(t)┘+E[N0(t)]+ΔIN(t)+ΔN0(t),  eq. (4)
where E[ ] denotes mathematical expectation and where Δ denotes the variation around the mean. The fundamental problem can now be clearly seen. Since there are no measurements available in the RBS that are related to the neighbor cell interference, a linear filtering operation can at best estimate the sum E└IN(t)┘+E[N0(t)]. This estimated sum cannot be used to deduce the value of E[N0(t)]. The situation is the same as when the sum of two numbers is available. Then there is no way to figure out the values of the individual numbers. This issue is analyzed rigorously for the RoT estimation problem in [1] where it is proved that the thermal noise power floor is not mathematically observable.
The interference level in current cellular networks or radio communications networks is increasing fast today. This densification leads to new players entering the market. As a consequence, regulation on frequency separation between equipment emitting electromagnetic radiation is becoming more difficult to maintain. In some countries this regulation is already weak. Recently, operators in several countries have been experiencing problems with such interference of sources external to the cellular system, in this case the WCDMA system. The technical equipment causing this interference may be mobiles of other cellular networks, poor frequency planning, illegal radio equipment, distant radar transmitters, or distant TV transmitters, to mention a few.
The treatment of this interference varies depending on it's duration. In case the interference is of a duration comparable to the normal high traffic/high interference periods of the cells in question, no action can be taken. This follows since there are no measurements defined that can be used for separation of electromagnetic energy from neighbor cells and from sources external to the WCDMA system. In case the duration is significantly longer than the normal high traffic/high interference periods of the cells in question, it is reasonable to handle the interference as an increase of the thermal noise power floor of the cells in question, or equivalently an increase of the receiver noise factor of the RBS.
The effect on the performance of the uplink (UL) of the cell is a reduced throughput. This follows since there is an UL interference threshold above which a scheduler of the traffic is not allowed to go. This is exactly the same situation as when there is a lot of traffic in neighbor cells, this then reduces the amount of traffic that can be allowed in the own cell. The real problem is that if the in-band interference external to the WCDMA system persists for very long, then the throughput reduction may remain during all times of the day, a situation that is not acceptable for the operators.
The remedy to the problem, as stated above, is to accept the interference as an increase of the thermal noise power floor. Since the allowed interference threshold of the scheduler is effective relative to the thermal noise power floor, the throughput reduction in the UL is removed.
Due to reasons explained below, the thermal noise power floor (level) needs to be estimated in the RBS. This estimator encounters two major problems in case of in-band interference external to the WCDMA system. These coupled problems are:                The thermal noise power floor estimate needs to be maintained with high precision during “normal” periods of high traffic and also in situations with in-band external interference. The prior art algorithm is not robust enough to anticipate very high levels of interference in HetNets. This may cause the estimates of the thermal noise power floor to become too high during normal operation, which affects the planned performance of the UL of the cell negatively;        During very long duration of in-band external interference the thermal noise power floor estimate is allowed to increase, thereby removing the throughput reduction associated with the in-band interference. However, when the in-band interferer is turned off, most often this is an instantaneous turn-off, the interference drops. A remaining high thermal noise power floor estimate then allows severe over-scheduling, since the threshold is relative to the estimated thermal noise power floor. This is not acceptable. Hence the thermal noise power floor estimate needs to drop immediately, to the level without in-band interference. The prior art estimator is however not accurate enough immediately after such a negative transient change. This also affects the planned performance of the UL of the cell negatively.        
Prior art noise floor tracking algorithms are somewhat prepared for noise floor tracking, allowing for delayed tracking of thermal noise power floor steps caused by longer term in-band interference. The input power scaling, see below, is a crucial ingredient for this. This is illustrated by FIG. 1. The time constant of the thermal noise power floor estimation algorithm is set to 40 h. The operation is as intended, however as can be seen in the FIG. 1 the standard deviation of the estimate increases immediately, causing the actual estimate to become more and more noisy. Power is defined along a y-axis and time is defined along an x-axis.
A problem may hence sometimes occur, e.g. when in-band interference steps occur, in that the thermal noise power floor estimate tends to become noisy, well before the actual step of the estimator is set to occur. This is illustrated in FIG. 1 in circled sections.
Moreover, when a large negative thermal noise power floor step occurs, the prior art algorithms do reduce the thermal noise power floor immediately. The consequence is that the thermal noise power estimate remains noisy for quite a long period of time, due the scaling of the standard deviation of the estimate. This results in a non-accurate estimation leading to a reduced performance of the radio communications network.